This invention relates to an injection molding machine for controlling position, back pressure and speed of a screw disposed in a heating cylinder of the injection molding machine with high precision during a material resin measuring process and also relates to a method for controlling the injection molding machine for the same purpose.
In designing an injection molding machine particularly by taking the automatic operations or controls of the various steps into consideration, attentions should be paid to the improvement of quality of products, the energy-saving and the improvement of productivity, and these attentions should be paid particularly during the material resin measuring process. Regarding a point of the improvement of the quality of the products, since the quality thereof is mainly affected by the injection speed, pressure, resin temperature, injection amount and the like, it is required to measure a precise amount of the resin to be injected with high precision. Regarding a point of the energy-saving, the material resin in particle form fed into a heating cylinder of an injection molding machine is heated, sheared and kneaded therein by a screw to obtain an evenly molten resin and the energy required for the resin measuring stroke is minimized by effectively controlling the operation of the screw in conformity with the rate of revolution (called revolution number hereinafter), the back pressure of the screw, the kind or type of the resin and the temperature of the resin to be melted. Moreover, the productivity of the mold products can be achieved by minimizing the time interval required for the measuring process.
Generally, in an injection molding machine, a material resin is fed into a heating cylinder through a hopper and the resin fed into the heating cylinder is fed forwardly by rotating a screw. During this operation, the resin is heated by a heater such as heating coil located on the outer periphery of the heating cylinder and then sheared and kneaded by rotating the screw. When the resin fed into a space formed at the front end portion of the heating cylinder is injected into a mold through a nozzle formed at the front end of the heating cylinder, the screw is then slightly retracted by the pressure of the resin injected into the mold, thereby preventing the molten resin from flowing out outwardly. At this time, a back pressure is applied to the screw by a drive mechanism operatively connected to the screw thereby preventing the introduction of air into the heating cylinder through the nozzle and the hopper and measuring the resin amount precisely. Accordingly, the screw is gradually retracted by the pressure difference between the resin pressure and the back pressure applied by the drive mechanism. The revolution number and the back pressure of the screw are preset experientially by the kind and temperature of the resin to be used, and the position of the screw in the cylinder for determining the resin amount for injection is set by a detecting means such as a limit switch, which is disposed in the drive mechanism, as well as means for rotating the screw and means for applying the back pressure to the screw.
With a construction of the injection molding machine of conventional type described above, the position of the screw is detected by the limit switch and accordingly controlled indirectly by the relative relationships between the revolution number of the screw and the back pressure thereof. This involves such a problem that the screw may stop at a position beyond the desired stop position for the reason that the rotation of the screw is stopped after the operation of the limit switch. In order to obviate this defect in the prior technique there is no countermeasure other than to gradually slow down the rotation of the screw as the screw approaches the limit switch, or to set the operating point of the limit switch before the theoretical operating point by taking into consideration the excessive backward movement of the screw. For the reason described above, it is necessary in actual to determine the screw position during the resin measuring stroke by repeating the trials and errors. Furthermore, it is complicated to precisely set the operating point of the limit switch by external causes or disturbances such as kinds of resins to be used, shapes of molds, temperature variation, moisture of the resin and the variation of the resin amount fed from the hopper into the heating cylinder, thus being difficult to obtain the accurate resin amount for injection as well as the setting of the operating point of the limit switch. It is, therefore, impossible to evenly inject the resin into the mold and to prevent the degradation of the quality of the mold products. Moreover, in this conventional technique, it is obliged to unnecessarily reduce the revolution numbers of the screw even in the normal operation to stop the screw at the accurate position, thus being required for measuring the resin amount in a short time and obtaining high energy efficiency as well as the improvement of the rate of production. The vibration or overshoot of the screw in the resin measuring process due to the disturbances as described before adversely applies unnecessary compression or reduction of the pressure to the resin during the measuring process and causes the molten resin to be flowed out from the nozzle or reversely into the hopper. Accordingly, it is required to adaptively control the injection molding machine in response to the speed and position of the screw without affecting the behavior of the screw even in a variable operation of the control system of the injection molding machine.